Treatment of kraft black liquor and product



TREATMENT OF KRAFT BLACK LIQUOR AND PRODUCT Filed sept. 1o, 1958TREATMENT oF KRAFT BLACK LIQUoR AND PRODUCT` Frank J. Ball and RichardPueschel, Charleston, S.C., as-

sgnors to West Virginia Pulp and Paper Company, New York, N Y., acorporation of Delaware Filed Sept. 10, 1958, Ser. No. 760,185

8 Claims. (Cl. 260-124) This invention relates to a process for treatingresidual black liquor from the kraft pulping process whereby dimethylsulfide is produced, lignin contained in the black liquor is modifiedand the aliphatic acid content in the black liquor is substantiallyincreased.

it is well known from the teachings of Hagglund et al. in Reissue PatentNo. 24,293 and Cisuey et al.A in Patent No. 2,816,832 that dimethylsulfide can be produced in substantial quantities from residual blackliquor of kraft pulping processes. According to these teachings,however, in order to obtain any appreciable amounts of dimethyl sulfideit is necessary that sulfur in certain forms be added to the blackliquor. The sulfur is added by Hagglund et al. in the form of inorganicsulfidic sulfur, i.e. sulfides or bisultdes, while Cisney et al.utilizes elemental sulfur for addition to the black liquor wherein theelemental sulfur forms sulfides in reaction with alkali present in theblack liquor.

We have found, however, that if black liquor from kraft pulpng nothaving sulfur in any form added is heated to 220 to 350 C. at a pressurenot less than thevapor pressure of the `black liquor at the existingtemperature and maintained in that temperature range for a period oftime varying with the temperature that the yield of dimethyl sulfideobtained is substantially the saine or greater than the yields obtainedby practicing the methods of Hagglund and Cisney. We have also foundthat the production of dimethyl sulfide during this heating isaccompanied by the dimethylation of lignin and the formation of certainlow molecular weight aliphatic acids.

During heating of kraft black liquor at temperatures between 220 and 350C. at a pressure not less than the vapor pressure of the black liquor atthe existing temperature, whatever sulfides or bisulfides may be presentin the liquor are little affected by the'treatment. The sulfuringredient of the dimethyl sulfide produced is chiefly supplied by othersulfur compounds present in the waste liquor. It has been found,however, that while it is not necessary to have any sulfidic sulfurpresent it is desirable to do so. Our experiments indicate thatconsiderable quantities of dimethyl sulfide can be produced from kraftblack liquor which does not contain any sulfide or bisulde but in whichthe sulfidic sulfur has been oxidized for example prior to concentratingto avoid sulfur losses in that operation. In these cases, however,

although substantially the same quantities of dimethyl ,sulfide may beformed as when sulfidic sulfur is present,

theV formation of dimethyl rsulfide proceeds at a very slow rate. Whensome sulfidic sulfur is present therrate g of formation of dimethylsulfide is greatly increased.

Patented Mar. 21, 196.1

lalthough increasing the rate of vformation Vof dimethyl sulfide, doesnot, as mentioned hereinbefore, greatly affect the yield as most of thesulfur in the dimethyl sulfide comes not from the sulfidic sulfur butfrom other sulfur compounds, such as thiosulfates, sulfites, sulphates,etc. which are formed during digestion of wood or subsequent handling ofthe liquor, i.e. during washing, storage, evaporating, etc. For example,ya typical partially concentrated kraft black liquor from the pulping ofpine wood containing 4.27 g./l. of sulfidic sulfur was heated under theprocess of this invention. The equivalent of V7.24 g./1. of sulfur wasrecovered inthe dimethyl sulfide while the black liquor still contained3.08 g./l. of sulfidic sulfur. Y Y

During the heating of the black liquor methoxy groups are removed fromthe lignin. These methoxy groups react with the sulfur compounds withinthe liquor to form the dimethyl sulfide. Besides beingdemethylated thelignin also undergoes other changes which are not fully understood butwhich result in a substantial increase kin the concentration of certainlow molecular weight aliphatic acids, such as acetic, propionic,glycolic, oxalic, and lactic acids, which acids will be referred to asaliphatic acids throughout the disclosure and claimsof thisspecification, and a considerable reduction in the pH of the liquor.These changes and their `significance will Vbe more fully shown afterthe following descriptionl of the complete process of this invention.

The complete process of this inventionris shown in the f ydrawing whichforms a part of this specification. The

single figure of the drawing is a flow diagram of the process. Y

The process is carried out with black liquor from a .kraft cook, say ofpine pulp, Vas the sole starting .ma-

terial. This black liquor may be obtained from the digesters, thewashers, lthejevaporators or any source.

For several reasons however, it isA best that the liquor be concentratedto a solids content of greater than about These reasons are to reducethe heat require-V ficulties encountered in handling the liquor. In atypi' cal case a partially concentrated black liquor yfrom ,the kraftpulping of pine wood from which the talloil skimmings have been removedmay contain 30-l35% solids and have a sulfidic sulfur content of 4 ton6Vg./l.

calculated as the sulfide ion. This 'material isdelivered to a suitablereactor, either of a continuous .orfbatch type, Without modification oraddition of any chemicals .and is there heated to 220 to 350 C. at orabove the vapor pressure of the black liquor. The temperature ismaintained for atimersufiicienty to produceVv substantial quantities ofdimethyl sulfide or to efecta desired degree of demethylation of the1ignin.'. Thefrtimelrequired is variable dependent on the temperatureV'employed. To effect substantial production of dimethyl sulfide anddemethylation of lignin maintenance of the temperature for approximately1 minute to several hours will be required with the shorter period oftime being applicable to the use of higher temperatures and vice versa.Preferably temperatures between 280 and 320 C. will be used with time of1 to 50 minutes.

At the end of the desired heating period the pressure is relieved, theliquor being allowed to pass via coolers to a receiver Where it shouldarrive with a temperature near the boiling point of the black liquor atatmospheric pressure. As the presure is relieved the more volatilecomponents of the liquor particularly the dimethyl sulfide is flashedinto vapor.

The part of the liquor which remains. liquid is collected and dischargedunder suitable control to a filter, being aciditied enroute to the lterto cause precipitation of the lignin and liberation of the aliphaticacids from their salts. The precipitated lignin is then recovered byfiltration. The filtrate from which the lignin has been removed issubjected to a process, such as liquid extraction, for separating thealiphatic acids.

If desired a two stage acidification process may be used resulting insavings in acid. During the first stage the liquor is acidified to a pHof approximately 9.5 which will cause approximately 95% of the lignin tobe precipitated. After removing this precipitated lignin by filtrationthe liquor is further acidiied to precipitate the final quantities oflignin and to produce the aliphatic acids from their salts.

The vapors and gases ashed from the liquor are conducted to a coolerwhere the noncondensible gases, such as carbon dioxide, are removed andthe condensable dimethyl sulfide and water vapor condensed. The waterand dimethyl sulfide, being practically mutually immiscible, form twolayers with the dimethyl sulfide floating on the aqueous condensate. Thedimethyl sulfide is separated from the water by decantation to obtain acrude dimethyl sulde. Pure dimethyl sulfide 4is then obtained bydistillation of the crude dimethyl sulfide.

While effecting as great or greater yield of dimethyl sulfide as thepreviously known methods of Hagglund and Cisney the present method hasseveral important advantages. The production of dimethyl sulfide isaccompanied by very little if any production of methyl mer-captan.

This eliminates the need for conversion of the methyl mercaptan todimethyl sulde or its removal from the dimethyl sulde. The addition ofsulfur compounds prior to heating, as in Hagglund and Cisney, increasesthe concentration of sulfur in the liquor thus favoring the reaction ofa single atom of sulfur combining with a single methoxy group to formthe mercaptan rather than combining with two methoxy groups -to formdimethyl sulfide.

A very significant effect of the reaction as carried out according tothis invention in relation to the recovery of lignin and aliphatic acidsis the lowering of the pH. The pH of the liquid fraction has been foundto decrease as much as 2.5 pH units during the heating of the liquor. Ahigher percentage of the lignin may thus be precipitated with the sameaddition of acid now used for precipitating lignin from untreated blackliquor or the same percentage of lignin may be precipitated with alesser addition of acid. In either case an economy is realized. Alsomore aliphatic acids are liberated with the same quantity of acid.

It is apparent that some of the carbohydrate material breaks down duringthe heating to yield aliphatic acids, since the concentration of theseacids is increased by a significant amount, up to Besides beingdemethylated the lignin contained in the liquor undergoes certainchanges which result in the production of a novel type of lignin. Whilethese changes are not fully understood the modified lignin produced bythe process of this invention possesses characteristics which make it ofmuch greater value than normal lignin.

fl- For example, the moist sodium salt of this modified lignin has beenfound to be a very effective oxygen-scrubber. In most applications wherenormal lignin is used this modified lignin may also be used more easilyor effectively.

The chemical structure of the lignin produced by this process will varyconsiderably with the conditions under which the black liquor istreated. The modified lignin from kraft pulped pine will contain 0.1 to12% by Weight of methoxy groups compared to 13 to 15% for normal kraftpine lignin, 0.5 to 2% of sulfur compared to 1.0 to 3.0% for normalkraft pine lignin, and up to 50% more acidic groups than normal kraftpine lignin. Although not definitely established, it appears the ligninof this process has a wide range of molecular weights whereas normalkraft pine lignin has a very uniform molecular size.

Similar results are obtained using black liquor from the kraft pulpingof hardwoods. Hardwood lignin, however, contains a much higher initialmethoxy content than pine lignin. Thus hardwood lignin which containsapproximately 21% methoxy groups when demethylatcd may contain a higherpercentage of methoxy groups than normal pine lignin. For example insubjecting kraft liquors from the pulping of hardwoods to the process ofthis invention lignin containing up to 18% methoxy groups were produced.

vThe changes in the chemical structure of the lignin causes greatchanges in the physical and chemical characteristics of the lignin. Forexample, compared to normal kraft pine lignin, pine lignin modified bythis process has greater solubility in many organic solvents such asethanol, hexanol, methyl acetate, acetone, epichlorohydrine, dioxane,triethylphosphate, dimethyl sulphone, and mixtures such as 1:1methanol-benzene, increased reactivity with organic and inorganiccompounds such as formaldehyde and oxygen, a more spherical particleshape, and a much greater ether extractibility, up to 30% while normallignin has no ether extractibility.

The lignins from this process are greatly superior to lignins modifiedby the methods of Hagglund and Cisney which contain high percentages ofsulfur, up to 15%, and have little if any solubility in most organicsolvents.

The modified lignins produced by the process of this invention will formsalts. All substances which will form salts with normal lignin will alsoform a salt with the modified lignin by the same processes. Forinstance, by precipitating the modified lignin from the black liquor athigh pH, about 9.0 to 10.0, the sodium salt is obtained. Other modifiedlignin salts can be obtained by the same method generally used inproducing normal lignin salts, i.e. by reacting lignin in solution at aproper pH with thc appropriate positive ion and precipitating the ligninsalt formed at a pH above that at which the salt will be decomposed.

From the many runs which have been made the following are chosen astypical.

Example 1 Concentrated black liquor from a kraft cook of pine woodcontaining 35 .35 solids after skimming off of the tall oil soaps washeated to 1) 220 C. and held at that temperature for (2) 36 minutesunder a pressure of (3) 800 p.s.i. This heating lowered the pH of theliquor from 11.90 to (4) 11.15. This liquor was ashed to remove gasesand vapors and the pH of the liquid fraction was reduced to 9.5 by theaddition of (5) 85 ml. of 31 B. sulfuric acid for precipitating themajor portion of the lignin. The liquor was filtered and acidifed againto precipitate the remaining lignin and to liberate the aliphatic acidsfrom their salts. The liquor was again filtered to remove the ligninprecipitated and the aliphatic acids were extracted. The total yields ofdry lignin and pure dimethyl sulfide expressed as percentages of blackliquor solids were, respectively, (6) 34.5 and (8) 0.22. The percentagereduction of the methoxy groups on the lignin was (9) 12%. The aliphaticacid yield expressed in i i ,n

grams per liter of the liquor heated was (7) 24.8. The procedureconformed generally to the flow diagram and to the description inconnection therewith.

:The details of Example l are tabulated below together 6 vthe order ofone-quarter hour totwo hours while maintaining it under a pressure whichis at all times at least las great as the vapor pressure of the blackliquor at the existing temperature of the black liquor, therebyproducwith corresponding details of Examples 2 to 8. In Ex- 5 ing aquantity of dimethyl sulfide whose lsulfur content is amples 2 through 8the starting material and procedure derived chiefly from the initiallynon-sulfidic sulfur and were the same as for Example l with theexception of the exceeds the total initial sulfidic sulfur content ofthe black indicated variations. liquor, an increased yield of lowmolecular weight alidemand Lig-nin Aliphatic DMS Percent Example Temp.,Time, Pressure, Dep. perliter Yield, acid Yield, Red.

O. Min. p.s.i. pH to pH 9.5 Percent Yield Percent OCHs m1. 31 BLS BLSBeHrso,

a6 soo 11. 15 85 34. 5 24. s 0. 22 12 14 soo 1o. 75 e5 33. 9 1. as 33 7a900 1o. 7 55 29. 1 25. s 2.21 89 15 950 10.2 55 3o. s 1s. 2 2. e3 es ao95o 9. 9 2o 29. s 25.9 2.81 72 s 17 1, 350 9. 7 30 2s. 4 22. s a. 08 94Control None None 11. 9 55 34. 9 2o. 9 o. o o

While certain preferred embodiments of the invention phatic acids, and amodified lignin product, and recoverhave been illustrated and describedin detail, it is to be understood that changes may be made therein andthe invention practiced in other forms. It is not therefore, theintention to limit the patent to the specific details illustrativelydescribed but to cover the invention broadly in whatever form itsprinciples may be utilized.

We claim:

l. The method of treating residual black liquor of the kraft pulpingprocess having a small initial ratio of sulfldic sulfur to total solidsinherently present and a considerably larger initial ratio of chemicallycombined, nonsulfidic sulfur to total solids inherently present, whichcomprises heating such black liquor, concentrated to a solids content of25 to 60%, to, and maintaining itin, the range of 220 to 350 C. under apressure which is at all times at least as great as the vapor pressureof the black liquor at the existing temperature of the black liquor,thereby producing a quantity of dimethyl sulfide whose sulfur content isderived chiefly from the initially non-sullidic sulfur of the blackliquor, and separating and recovering from the back liquor the dimethylsulfide so produced.

2. The method of claim l in which a substantial portion of the suliidicsulfur is oxidized prior to the concentration of the black liquor.

3. The method of treating residual black liquor of the kraft pulpingprocess having a small initial ratio of suldic sulfur to total solidsinherently present and a considerably larger initial ratio of chemicallycombined, nonsuliidic sulfur to total solids inherently present, whichcomprises heating such black liquor, concentrated to a solids content of25 to 60%, to, and maintaining it in, the range of 220 to 350 C. under apressure which is at all times at least as great as the vapor pressureof the black liquor at the existing temperature of the black liquor,thereby producing a quantity of dimethyl sulfide whose sulfur content isderived chiefly from the initially non-suldic sulfur of the blackliquor, an increased yield of low molecular weight aliphatic acids, anda modified lignin product, and recovering from t-he black liquor adesired product so produced.

4. The method of treating residual black liquor of the kraft pulpingprocess having a small initial ratio of sulfidic sulfur to total solidsinherently present and a considerably larger initial ratio of chemicallycombined, nonsuliidic sulfur to total solids inherently present, whichcomprises heating the black liquor, concentrated to a solids content of25 to 60%, to 220 to 350 C. and maintaining it in that temperature rangefor a period of ing from the black liquor a desired product so produced.

5. The method of treating residu-al black liquor of the kraft pulpingprocess having a small initial ratio of sulfidic sulfur to total solidsinherently present and a considerably `larger initial ratio ofchemically combined, nonsulidic sulfur to total solids inherentlypresent, to produce and recover dimethyl sulfide, a modified lignin, and

. an increased yield of aliphatic acids, which method cornprises heatingthe black liquor, concentrated through evaporation to a solids contentyof 25 to 60%, to the range of 220 to 350 C. `and maintaining it in saidrange for a period of the order of one-quarter hour to two hours, whilemaintaining it under pressure which is at all times `at least as greatas the vapor pressure of the black liquor at the existing temperatureIof the black liquor, thereby producing dimethyl sulfide whose sulfurcontent is derived chiefly from the initially non-suliidic sulfur andexceeds the initial sulfidic sulfur content of the black liquor,reducing the pressure on the heated black liquor to a pressure notsubstantially above atmospheric pressure to cause flashing of vaporsfrom the black liquor at a temperature not substantially above theatmospheric boiling point of the black liquor, separating the flashedvapors from the liquid fraction, acidifying the liquid fraction to causethe precipitation of the lignin and conversion of aliphatic acid saltsinto aliphatic acids, filtering out the lignin and extracting thealiphatic acids from the filtrate, condensing the water and dimethylsulfide vapors flashed from the liquid fraction, and separating thecondensed dimethyl sulfide from the condensed water.

6. A modified lignin product, obtained from black liquor from the kraftprocess of pulping wood having a small initial ratio of sullidic sulfurto total solids inherently present and a considerably larger initialratio of chemically combined non-sulidic sulfur to total solidsinherently present, by heating such black liquor, concentrated to asolids content of 25 to 60%, to, and maintaining it in, the range of 220to 350 C. under a presprecipitate the modified lignin product therefrom,said modified product being characterized by having a1 sulfur contentbetween 0.5 and 2.0% by weight and having,

when compared to the kraft lignin from which the modi-v fied lignin wasprepared, a substantially reduced methoxy content, greater acidic groupcontent, greater etherexf I Y tractable content, increased reactivitytoward oxygen and y i formaldehyde, and substantially increasedsolubility in each of acetone, hexanol, and 1:1 mixture ofmethanolbenzene.

7. A modified lignin product as set forth in claim 6, wherein the blackliquor s from the kraft process of pulping hardwood.

8. 'A modied lgnn product as set forth in claim 6, wherein the blackliquor s from the kraft process of pulpng pine wood, and wherein themethoxy content is reduced from 12 to 94% as compared with the unmodiedproduct.

u References Cited in the le of this patent UNITED STATES PATENTSHagglund Mar. 19, 1957 Rinman Aug. 9, 1927 Salvesen Aug. 25, 1950 BirdApr. 25, 1950 FOREIGN PATENTS Canada July 18, 1950

1. THE METHOD OF TREATING RESIDUAL BLACK LIQUOR OF THE KRAFT PULPINGPROCESS HAVING A SMALL INITIAL RATIO OF SULFIDIC SULFUR TO TOTAL SOLIDSINHERENTLY PRESENT AND A CONSIDERABLY LARGER INITIAL RATIO OF CHEMICALLYCOMBINED, NONSULFIDIC SULFUR TO TOTAL SOLIDS INHERENTLY PRESENT, WHICHCOMPRISES HEATING SUCH BLACK LIQUOR, CONCENTRATED TO A SOLIDS CONTENT OF25 TO 60%, TO, AND MAINTAINING IT IN, THE RANGE OF 220* TO 350*C. UNDERA PRESSURE WHICH IS AT ALL TIMES AT LEAST AS GREAT AS THE VAPOR PRESSUREOF THE BLACK LIQUOR AT THE EXISTING TEMPERATURE OF THE BLACK LIQUOR,THEREBY PRODUCTING A QUANTITY OF DIMETHYL SULFIDE WHOSE SULFUR CONTENTIS DERIVED CHIEFLY FROM THE INITIALLY NON-SULFIDIC SULFUR OF THE BLACKLIQUOR, AND SEPARATING AND RECOVERING FROM THE BACK LIQUOR THE DIMETHYLSULFIDE SO PRODUCED.
 6. A MODIFIED LIGNIN PRODUCT, OBTAINED FROM BLACKLIQUOR FROM THE KRAFT PROCESS OF PULPING WOOD HAVING A SMALL INITALRATIO OF SULFIDIC SULFUR TO TOTAL SOLIDS INHERENTLY PRESENT AND ACONSIDERABLY LARGER INITAL RATIO OF CHEMICALLY COMBINED NON-SULFIDICSULFUR TO TOTAL SOLIDS INHERENTLY PRESENT, BY HEATING SUCH BLACK LIQUOR,CONCENTRATED TO A SOLIDS COSNTENT OF 25 TO 60%, TO, AND MAINTAINING ITIN, THE RANGE OF 220* TO 350*C. UNDER A PRESSURE WHICH IS AT ALL TIMESAT LEAST AS GREAT AS THE VAPOR PRESSURE OF THE BLACK LIQUOR AT THEEXISTING TEMPERATURE OF THE BLACK LIQUOR, AND SEPARATING THE MOFIFIEDLIGNIN PRODUCT FROM THE BLACK LIQUOR BY ACIDIFYING THE LIQUOR TOPRECIPITATE THE MOFIFIED LIGNIN PRODUCT THEREFROM, SAID MODIFIED PRODUCTBEING CHARACTERIZED BY HAVING A SULFUR CONTENT BETWEEN 0.5 AND 2.0% BYWEIGHT AND HAVING, WHEN COMPARED TO THE KRAFT LIGNING FROM WHICH THEMODIFIED LIGNIN WAS PREPARED, A SUBSTANTIALLY REDUCED METHOXY CONTENT,GREATER ACIDIC GROUP CONTENT, GREATER ETHER EXTRACTABLE CONTENT,INCREASED REACTIVITY TOWARD OXYGEN AND FORMALDEHYDE, AND SUBSTANTIALLYINCREASED SOLUBILITY IN EACH OF ACETONE, HEZANOL, AND 1:1 MIXTUREMETHANOLBENZENE.